The fetch buffer is a memory location in a client-server environment where cursor data received from the server and held before transmit to the application. Setting a bigger fetch buffer size can significantly improve query performance by reducing network traffic. It is the amount of data that client accepts from the server without waiting to send a "received" message which make the difference. Starting with Informix version 11.70.xC5, you can set the maximum size of the fetch buffer to 2 GB to increase query performance.
The FET_BUF_SIZE environment is used to set the size of the application fetch buffer. This environment variable can override the default setting for the size of the fetch buffer for all data types except BYTE and TEXT values. An ANSI database needs addition consideration; you must set transactions to READ ONLY mode for the FET_BUF_SIZE environment variable to improve performance, otherwise rows are returned one by one.
A valid FET_BUF_SIZE setting is in effect for the local database server and for any remote database server from which you retrieve rows through a distributed query in which the local server is the coordinator and the remote database is subordinate. The greater the size of the buffer, the more rows can be returned, and the less frequently the client application must wait while the database server returns rows. A large buffer can improve performance by reducing the overhead of filling the client-side buffer.
In case of an invalid FET_BUF_SIZE setting, Informix not generates any error. It uses the default fetch buffer size.
The combination of fetch buffer, communication buffer (the "b" option in the sqlhosts file), network driver size and application cursor can enhance the query performance farther.